To perform laser or photon material processing it is often desirable to create a field of uniform irradiation so that the process will produce consistent results within an area of a homogenous optical field. An unrequited goal of current technology is to create a beam homogenizer device capable of transforming a non-uniform light input into a field of homogenized illumination of, for example, rectangular shape with a high aspect ratio. In this context, a high aspect ratio rectangular field is a field having one dimension in the X direction/axis greater than three (3) times the dimension in the (perpendicular) Y direction/axis (or visa versa). High aspect ratio illuminated fields are also known as “long line” fields of illumination.
Long line fields of uniform illumination are useful in many applications such as, for example, photolithography, laser micromachining, laser annealing and other laser or optical surface treatment. Often, a uniformly illuminated mask is imaged using homogenized light onto a target using a projection lens. The mask may be scanned through a uniform field in Coordinated Opposing Motion (COM) to create large patterns in the target plane.
To produce a homogenized field of illumination, beam homogenizers are used. Beam homogenizers are devices that modify an incoming illumination beam such that the uniformity of the output illumination field is largely independent of the illumination uniformity of the raw laser beam (i.e., the beam is homogenized). When non-uniform laser illumination is input into a homogenizer, the beam is angularly or spatially (or both) mixed in a manner that allows a uniform field of illumination to be produced at the output.
There are several prior art methods of fabricating a beam homogenizer. For instance, a multi-lenslet array, known as fly's eye homogenizer, can be used to form a uniform homogenized illumination field. Another method utilizes a multi-segmented reflective or refractive surface to overlap portions of the incident beam to produce a uniform field. Yet another widely used homogenizer technique utilizes the kaleidoscope principle. Holographic devices may also be utilized to create uniformity and control beam divergence.
The prior art devices encounter difficulties creating simple and cost-effective high aspect ratio homogenized fields of illumination. For example, using a simple fly's eye device, to create a 5:1 aspect ratio field homogenized in the x and y directions, the dimensional aspect ratio of the individual fly's eye elements (i.e., ratio of element height to width) must be 25:1 in order to insure field uniformity to approximately ±5%. For 5% uniformity with a 100:1 aspect ratio homogenized field, the optical elements will have an aspect ratio of 500:1. Optical parts with such high aspect ratios are extremely difficult to manufacture. Although such mechanical aspect ratios can be moderated by introducing an anamorphic imaging device between the fly's eye and the mask, such an addition increases complexity to the system.
The nature of the non-uniformity of the raw beam dictates the number of homogenization elements that must be utilized in a given direction. For instance, for a fly's eye array with a 10×10 element array (or 100 elements), each element contributes 1% of the total power to the homogenized field. If the raw input beam possesses a 20% linear non-uniformity, then the aperture of each lenslet will be illuminated uniformly to better than 2% and the homogenized beam will be uniform to better than 2%. For a typical excimer laser application requiring +/−5% homogenization, empirical rules dictate the utilization of at least 6 elements per direction. Thus, for a 1:1 aspect ratio field, at least 36 fly's eye elements are required.
Moreover, the higher the aspect ratio, the more fly's eye homogenizer elements in the lens array are required for good homogenization. For example, to produce an illumination field having an aspect ratio of 5:1, and if the empirical rule for such a system designates 6 elements per direction, then a lens array of 180 elements are required to produce the homogenous optical field.
FIGS. 1A–1D illustrate this problem. For example, FIG. 1A illustrates a fly's-eye array required to create a uniform, 6×6 square field (i.e., aspect ratio of 1:1). If the empirical rule requiring 6 segments per direction is obeyed, the array will contain 36 elements. In FIG. 1B, a 2:1 aspect ratio field has been created. This geometric constraint requires that the number of elements in the vertical direction double and since the horizontal direction requires at least 6 elements for uniformity, 12 are needed for the vertical (i.e 6×12=72 total lenses). In FIG. 1C the problem gets worse—to create a 10:1 aspect ratio field, 60 segments are required in the vertical direction resulting in the total number of elements equaling 6×60=360. FIG. 1D illustrates the problem as it approaches a line field limit. In this case, 3600 elements are required (6×600). It becomes obvious to one of skill in the art that this design becomes prohibitively expensive and impractical to fabricate for this high aspect ratio beam.
Other problems exist for kaleidoscope homogenizers. To produce uniformity in two directions/axes (d1 and d2), a high aspect ratio forces the length of the kaleidoscope homogenizer to become exceedingly long, since the length of the kaleidoscope is proportional to the longest dimension (d1) of the homogenized field. FIGS. 2A through 2C illustrate this problem. For example (FIG. 2A), in a square field kaleidoscope homogenizer, the length of the kaleidoscope controls how many times the light is segmented. For example, for a square kaleidoscope with a given input dimension of, say, 1 cm, and an input cone angle of, say 5 degrees, the light beam may reflect three (3) times and the cone angle will be segmented 7 times per direction. This will typically produce good homogenization. For such a case, the kaleidoscope length will be 34.3 cm. However, for a 2:1 aspect ratio kaleidoscope (FIG. 2B) with the same short dimension of 1 cm, the 3 reflection condition requires the kaleidoscope to become twice as long (68.6 cm) since the long dimension is 2 cm and more path is required to achieve the segmenting. Now, for a 10:1 aspect ratio system (FIG. 2C) with a 1 cm short axis dimension, the length of the kaleidoscope becomes 343 cm. A 100:1 case is worse yet, resulting in a length of 3430 cm (or about 100 feet!). It becomes obvious to one of ordinary skill in the art, that for a high aspect ratio homogenizer, such a kaleidoscope design becomes impractical to implement and extremely cost prohibitive.
Furthermore, to achieve a high aspect ratio field with good laser coupling efficiency, one might expand the laser beam in one direction and compress it in the other. Since the divergence of the beam changes inversely proportionally to the expansion ratio, the divergence will increase beyond the acceptance numerical aperture (NA) of the lens if a great deal of compression is performed. Such homogenization system results in poor light utilization efficiency.
Because of the above noted problems, current long line illumination fields are implemented with only single axis homogenization. In some cases, the long axis would be homogenized while the short axis is left as spatially non-uniform as the original beam spatial intensity distribution in the relevant direction. In other cases, the full length cannot be accommodated, because the long axis of the field is not sufficiently uniform. In these applications, the laser beam is truncated so that only the more uniform section of the beam is used. This truncation lowers efficiency of the beam delivery system.